Focused solar energy collection system to increase efficiency and decrease cost

ABSTRACT

An efficient and cost effective system for the collection, concentration, and delivery of disperse solar energy to a central location for use in electrical power generation. The system includes a unique structural design for a strong lightweight parabolic dish heliostat. It couples this heliostat with a small collimating lens (or mirror) and a flat side mirror to redirect the resultant concentrated solar energy beam to a central receiver.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the provisional applications:

-   -   1) Ser. No. 61/280644 110609, filed 2009 Nov. 6

FEDERALLY SPONSORED RESEARCH

Not applicable

SEQUENCE LISTING OR PROGRAM

Not applicable

FIELD OF INVENTION

This invention pertains to the collection, concentration, and deliveryof disperse solar energy to a central location for use in electricalpower generation. It describes an efficient and cost effective means ofdoing so.

BACKGROUND

Solar thermal energy is gradually gaining attention as a better way ofsupplying renewable clean energy. The technology described in thispatent is applicable to large scale solar power production in giga-wattquantities. A power tower configuration is one form of large scale solarthermal collection facility. It consists of a field of flat mirrors(heliostats) on the ground, each aimed so as to redirect the sunsscattered rays to shine upon a central (receiver which is mounted on acentral tower). The Barstow facility (Solar 1 and 2) is an example ofthis type of facility. The heat collected by the receiver can be used todrive a power generation unit, typically a steam turbine, or can bestored for later use, thereby extending the power output capability pastdaylight hours.

While this scheme has its advantages, there are still severalinefficiencies to this configuration in its current form. The largestinefficiency is the cosine effect. Sunlight falls on the earth with afixed amount of energy per unit area facing the suns rays. Dependingupon where a mirror is positioned on the ground, it may have to beplaced at a considerable angle to the suns rays in order to reflect thesuns rays directly at the receiver. This angle reduces the amount ofsunlight the mirror is able to collect by a factor equal to the cosineof the angle. This factor can be as much as 25%. Consequently eachmirror must be oversized to accommodate its worst angle condition.Typical heliostat arrangements try to circumvent this inefficiency byplacing the mirrors to the side of the tower opposite the sun whichreduces the reflection angle. Yet a substantial cosine effect is alsoproduced by the suns relative change in position in the sky throughoutthe day. So the brunt of this effect cannot be avoided by simply skewingthe heliostat distribution.

The heliostats should be arranged so as to deliver maximum energy to thereceiver. However, the attempted circumvention of the cosine effect byskewing the mirrors to one side of the receiver exacerbates otherinefficiencies. Scattering of the reflected rays; due to solar beamdivergence, mirror optical imperfections, atmospheric considerations, ordue to aiming error can be minimized by placing the mirrors closer tothe receiver. Each of these causes introduces error in the reflectedangle of the sunlight, resulting in a portion of the reflected lightmissing the receiver. Since the error is angular, it gets worse withdistance. Thus scattering effects are minimized with a mirrorconfiguration that is concentric (closest) to the receiver. Theshadowing effect—where one heliostat casts its shadow on a neighboringheliostat—also becomes worse with distance. This is because eachheliostat must reflect sunlight over the heliostats between it and thereceiver. So the further from the receiver, the more land is requiredper heliostat.

The shadow effect problem may be ameliorated to some extent by makingthe tower higher. However, tower costs become more expensive than linearwith height and this can be prohibitive. This is driven in part by theweight of the receiver apparatus at the top of the tower, because thereceiver must be sized to the dimensions of each flat mirror panel onthe heliostats and it must have some thermal mass to reduce thesusceptibility to overheat. Attempting to reduce tower costs by using adown mirror at the tower top to direct the incoming sunlight to areceiver at ground level becomes extremely complicated to orchestrate.If flat mirrors are used on the ground, then the down mirror on thetower must consist of many flat facets; one for each mirror panel on theground and just as large. So the down minor doubles your mirror area andresults in an excessively huge down mirror. As result, thisconfiguration has been largely avoided.

Use of a parabolic down mirror would require parabolic up mirrors,because each up mirror must converge its light to the down mirrors focalpoint to avoid the light being scattered by the down mirror. Butparabolic up mirrors are not conducive to large angle reflections. Anangle of incidence that is not parallel to the up mirrors parabolic axisof revolution will result in scatter losses and changes in focal length.These scatter losses are then magnified by the added distance the lighttravels—tower to the ground. One additional but significant disadvantageto using parabolic heliostats in this fashion is that the shape of theparabola must vary with distance from the receiver in order to match thefocal length to that distance. So each heliostat is unique to itsposition, which drives up the cost.

We have attempted to present a brief, but fairly complete picture ofsome of the difficulties faced by the designer of a Power Tower solarthermal system. This patent solves many of those difficulties byintroducing a system that eliminates the cosine effect on the heliostatand minimizes scatter and shadow effects; thereby increasing systemefficiency and decreasing system cost.

SUMMARY

This system minimizes the cosine effect of traditional power towersystems. Parabolic heliostats are used, pointing always at the sun. Theparabolic shape converges the reflected sunlight upon a close coupledconcave quartz lens, placed with its focal point at the heliostat focalpoint in order to straighten the converging rays into a narrow beam ofconcentrated sunlight. The beam is reflected again by a mirror (onservos), redirecting it to a central receiver that is close to theground. Each parabolic heliostat is pointed directly at the sun,eliminating its cosine effect and utilizing its full reflectivepotential at all times. The outgoing beam is reflected (by a small flatmirror) directly towards a central receiver at a height slightly tallerthan the heliostat height. This last reflection is subject to the cosineeffect, but since the mirror is so small, the expense of over-sizing thesmall mirror is minimal. Because each heliostat is pointed directly atthe sun, the mirror distribution may be freely optimized to minimize thescatter effect and the shadow effect. More specifically, the heliostatscan be positioned concentric to the receiver; minimizing the distance tothe receiver in order to reduce scatter effect. The heliostats can alsobe placed closer together since the only consideration with respect toshadow effect is a clear path to the sun. Each heliostat directs itsenergy to the receiver at a height above its neighboring heliostats. Sothere is no limiting (clear path to the tower) consideration which wouldrequire heliostats that are further from the receiver to be spacedfurther apart from each other.

Furthermore the heliostat structure is very strong and lightweight dueto its wire braced truss construction. This is very important becausethe heliostat must be rigid for accuracy and lightweight for low costThe end result is an accurate, low cost system that maximizes the amountof sunlight that can be gathered from a given amount of real estate anddelivered to a central receiver. And it does so while minimizing themirror area of its heliostats.

BRIEF DESCRIPTION (OF THE FIGURES)

FIG. 1 depicts a side focusing heliostat in isometric view. A sectionedparabolic reflective dish is mounted atop an actuated post and supportedwith a unique wire frame bracing. A central shaft running along thedishes parabolic axis in combination with the parabolic dish itself andstrategically placed wires forms a light but sturdy truss configuration.A lens is positioned at the parabolic focal point and an actuated flatreflective side mirror is positioned on the heliostat axis just beyondthe lens. Magnified views are inset to provide detail of the actuatedparts.

FIGS. 2-6 are diagrams depicting the heliostat operation in profile. Thepath of the suns rays are traced using dashed arrows.

FIG. 2 depicts the basic heliostat configuration.

FIG. 3 demonstrates the ability of this heliostat configuration tocircumvent the cosine effect. Regardless of location (two positionsshown), each heliostat delivers its collected sunlight to a centralreceiver while being aimed directly at the sun. An equivalent flat plateheliostat is shown incomparison to demonstrate the disadvantages of thecosine effect (larger mirror area and unfocused beam delivery).

FIG. 4 depicts the solar beam divergence problem and demonstrates howincoming convergent sunlight becomes divergent when reflected orrefracted. This behavior (inherent to sunlight) affects the design ofthe heliostat, requiring optimum sizing of the reflective and refractivedevices.

FIG. 5 depicts an alternative variation on the basic claim whereby thecollimating lens is replaced by an hyperbolic reflector and the sidemirror is moved to a location closer to the parabolic mirror.

FIG. 6 depicts an alternative variation on the basic claim whereby thecollimating lens is replaced by an hyperbolic reflector and the sidemirror is moved to a location on the opposite (convex or back) side ofthe parabolic mirror. A hole in the center of the parabolic mirror isrequired in order to allow the focused beam a clear path to the sidemirror. This configuration may have some aiming advantages.

DETAILED DESCRIPTION

The system consists of multiple heliostats surrounding a centralreceiver placed close to the ground. Each Heliostat (shown in FIG. 1)consists of a parabolic mirror (1) mounted on a motorized stand (2). Thestand is equipped with actuators (2 a); providing a means of rotatingthe heliostat about the vertical and elevating the heliostats centralaiming axis (3) (the axis of the parabolic surface of revolution) toangles above or below the horizon. The lightweight support structure forthe mirror (1) consists of a central shaft (3 a) with integral hub (3 b)running along the central aiming axis (3) and an array of guy wires (4)attached to the ends of the central shaft ( 3 a) and to lugs embedded inthe mirror (1). The mirror (1) attaches to the central hub (3 b) alsovia lugs embedded in the mirror. The tensioned guy wires (4) place boththe central shaft (3 a) and the mirror (1) in compression, thus forminga rigid truss that holds the mirror (1) accurately parabolic. Theparabolic mirror (1) consists of a shaped structural core (such as balsawood) sandwiched between layers of fiberglass and lined with a mirroredsurface. Metal lugs are embedded in the fiberglass for attachmentpoints. Thus the mirror (1) is capable of sustaining the compressionforces of the truss. The mirror (1) can be fabricated in gore shapedpanels for ease of transportation. Vents at the panel join lines can beused to spoil wind loads.

A concave (diverging) quartz lens (5) is positioned perpendicular to thecentral aiming axis (3) with its focal point coincident to the focalpoint of the parabolic mirror (1). The lens is mounted on the end of thecentral shaft (3 a) using curved struts (3 c) that minimize shadowing.Surrounding the lens is a ring shaped actuator (6) which rotates a sidemirror mount (6 a). A planar side mirror (7) is mounted upon the sidemirror mount (6 a) and is tilted by a side mirror actuator (8). Theseactuators give two angular degrees of freedom to the side mirror (7) andprovide a comprehensive means to aim the planar mirrors (7) normal inany direction (using spherical coordinates).

Operation

Referring to FIGS. 1 and 2, the heliostat stand actuators (2 a) are usedto aim the heliostat directly at the sun. Aim is quantified in polarcoordinates by rotation about the axis of the heliostat stand (2) andelevation above the horizon. Aim is accomplished very precisely via wormgear actuators using stepper motors with built in encoders. For anygiven date, time, longitude and latitude, the aiming angles are readilycalculated using known astronomical relationships. Incoming solar rays,traveling parallel to the heliostats central aiming axis (3) arereflected from the parabolic mirror (1) towards the parabolas focalpoint to enter the lens (5) where they are collimated to travelapproximately parallel to the central aiming axis (3). At this point thesuns incoming rays have been collected into a concentrated beam ofsunlight traveling along the central aiming axis (3) directly towardsthe sun.

The concentrated sunbeam is then reflected by the planar side mirror (7)to travel horizontally to its target; the receiver. The two actuators(6) and (8) are used to move the planar side mirror (7) to aim the beamat the receiver. Actuator (6) rotates the side mirror mount (6 a) aboutthe central aiming axis (3). Actuator (8) rotates the planar side mirror(7) about the side mirror axis (7 a), an axis normal to the centralaiming axis (3) and in the plane of the planar side mirrors (7)reflective surface. Aiming of the side mirror is thus accomplished viatwo angles; about the central aiming axis and the side mirror axis.These angles are calculated values based on the dish aiming angles(actuators 2 a), heliostat geometry, and receiver location (and usingcoordinate transformations). These actuators (6) and (8) also consist ofworm gears driven be stepper motors with built in encoders, thoughsmaller in size than actuators (2 a). As the suns position in the skyvaries throughout the day, the heliostat stand actuators (2 a) mustrealign the parabolic mirror (1) to continuously target the sun andactuators (6) & (8) must realign the planar side mirror (7) to redirectthe outgoing beam at the receiver. For any given location on earth theangles required to accomplish the aiming are readily calculated based ondate, time of day, longitude, latitude, and receiver location.

A topic that should be addressed in gaging the worth of this inventionis solar beam divergence. The tremendous distance at which the sunresides relative to the earth lends sunlight the property of beingnearly parallel light. However, the sun is of finite size and as such,the light that it casts upon any given point on earth is a convergingcone of light that disperses upon reflection. The angle of this cone isvery small (0.55 degrees), but it does affect the accuracy of reflectionand refraction. Incoming sunlight that is not parallel to the centralaiming axis (3) of the dish will reflect off any point on the parabolicmirror (1) in an expanding cone of light until it hits the collimatinglens (5)—see FIG. 4. The sunlight will then be refracted through thelens (5) over a region of the lens that is centered at the point throughwhich a parallel beam would be refracted. Hence the problem of beamdivergence is magnified. Not only does the divergence angle propagatefrom parabolic mirror (1), through collimating lens (5), and bounced offof side mirror (7); non parallel light will refract through the wronglocation of the lens and consequently magnify the divergence. It isimportant to note that this problem becomes worse as the area of thedivergent light beam at the lens (5) approaches the diameter of the lensitself. The problem can be reduced by increasing the lens (5) size ordecreasing the distance from parabolic mirror (1) to the lens (5).

As the size of the lens (5) increases, its cost can become prohibitive,mainly due to the issue of weight. For this reason, several alternativeconfigurations are included in the claims. The lens (5) may be either astandard double convex lens or a fresnel lens to reduce weight. The lensmay also be replaced (see FIGS. 5 and 6) with a lightweight hyperbolicmirror (5 a) which reflectively collimates light from the parabolicmirror (1) back towards the parabolic mirror (1). In this latterconfiguration, the side mirror (7) is placed either between theparabolic mirror (1) and the hyperbolic reflector (see FIG. 5) or behindthe parabolic mirror (1) with an aperture placed in the center of theparabolic mirror (1) to clear a pathway for the beam (see FIG. 6). Thelens (5) or hyperbolic secondary reflector (5 a) may be modified to addconvergence to the outgoing beam in order to further ameliorate thedivergence issue.

Advantages

I. Since each parabolic mirror (1) is pointed directly at the sun at alltimes (see FIG. 3), the cosine effect is eliminated and the heliostatwill always operate at peak efficiency. Hence

-   -   a. The mirror (1) area need not be oversized to compensate a        cosine effect and this reduces system cost.    -   b. The heliostat placement need not be skewed to compensate a        cosine effect and this allows the most compact heliostat        placement pattern—concentric—and thus        -   i. reduces scatter loss by minimizing the average distance            of the heliostat to the receiver.        -   ii. Reduces real estate cost due to denser placement            distribution.

II. Because the parabolic mirror (1) is not focused on the receiver (buton a lens (5) in close proximity), its shape is independent of distancefrom the receiver. So all of the heliostat mirrors (1) can be madeidentical which reduces system cost.

III. The focal point of the parabolic mirror (1) is designed to be farenough from the mirror (1) to be above the tops of adjacent heliostatsat all times of daylight operation.

-   -   a. This enables the beam to travel directly to a receiver placed        close to the ground instead on top of an expensive tower. This        reduces or eliminates tower cost.    -   b. The narrow width of the beam and its height above all        parabolic mirrors (1) makes it easier to avoid shadowing effects        on light traveling from the heliostat to the receiver.        -   i. The beam only has to avoid being shadowed by other flat            mirrors (7) which are very small and easy to avoid.        -   ii. And by varying the distance of the side mirror (7) from            the lens (5), one can guide the beams of adjacent heliostats            to travel to the receiver at slightly different heights in            order to avoid shadowing.    -   c. The elimination of shadow effects from the heliostat to the        receiver reduces the placement constraints on the heliostat        enabling closer placement to the central tower, reducing scatter        loss and real estate cost.

1. A solar energy reflection and focusing system including: A device forreflecting and focusing solar energy onto a reactor or heat absorber forthe conversion of solar energy into thermal, chemical, electrical energyA primary parabolic dish for reflecting and focusing the solar energy Asecondary lens or reflector used to refocus the solar energy to modifythe solar beam convergence distance A tertiary reflector used to aim thesolar energy onto a reactor or down mirror
 2. The system of claim 1wherein the primary parabolic dish points directly at the sun throughoutthe day
 3. The system of claim 1 wherein the primary structure of theparabolic dish heliostat is a uniquely shaped structural truss composedof a. a central compression member running through the center of theparabolic dish (projecting both in front and back), b. radial tensionmembers composed of guy wires tying both ends of the central compressionmember to points on the parabolic minor, and c. the fiberglass sandwichbacked parabolic mirror itself acting as a compression member, bothradial and axial
 4. The system of claim 1 wherein the parabolic mirrorcan be segmented or single piece.
 5. The system of claim 1 wherein aquartz lens is attached near the focal point of the parabolic dish so asto approximately collimate the solar energy from the parabolic dish 6.The alternative to the system of claim 1 wherein the quartz lens isfabricated as a fresnel lens
 7. The system of claim 1 wherein the lenshas a rotating ring structure external to the diameter of the lens whichprovides a means to rotate a tertiary reflector surface about theparabolic dish axis so as to control the direction of the outgoing solarbeam when combined with claim
 8. 8. The system of claim 1 wherein thetertiary reflection surface has the capability to tilt about an axisperpendicular to the parabolic dish axis so as to control the directionof the outgoing solar beam when combined with claim
 7. 9. Thealternative to the system of claim 1 wherein the quartz lens is replacedwith a secondary reflector located inboard of the focal point of theparabolic dish so that the solar energy focuses across the area of thereflector surface
 10. The alternative to the system of claim 1 whereinthe secondary reflector (of claim 9) is an approximately hyperbolicshape so that the solar energy beam is more parallel to the axis of theparabolic dish after reflecting from the parabolic dish and thesecondary reflector
 11. The alternative to the system of claim 1 whereinthe lens (or secondary reflector of claim 9) produces a solar energybeam which has the required toe-in angle so that the solar energy beamreaches the reactor head or heat absorber surface at a reduced diameter12. The system of claim 1 wherein the tertiary reflector has anapproximately flat surface and is able to rotate about the primaryparabolic dish axis and tilt about an axis normal to the primaryparabolic dish axis so as to aim the solar energy beam to the reactorhead or heat absorber
 13. The system of claim 1 wherein the parabolicmirror surface is made from radially tapered structural segmentsattached at the segment edges to form a structural ring
 14. The systemof claim 13 wherein the parabolic mirror segments include vents at theedges in order to spoil any wind loads that the mirror may experience.15. The alternative to the system of claim 1 where the quartz lens ismade of commercially available glass materials
 16. A solar energyreflecting and focusing system wherein the helio-stat configurationoperates by continuously aiming at the sun thereby eliminating thecosine effect due to angular differences between heliostat aiming axisand the suns rays.
 17. A solar energy reflecting and focusing systemwherein the secondary lens or mirror in combination with the tertiaryreflector enables a continuous aiming of the solar energy beam at thereactor head or heat absorber; adjusting in response to the motion ofthe primary parabolic axis as it tracks the sun throughout the day